Metal pipe lined with non-metallic material, usually a thermoplastic substance, has been known for considerable time and is in widespread used for handling diverse materials spanning the gamut from corrosive acid to food. Many types of thermoplastics have been used for the liners. For example, polyvinylidene fluoride (PVDF), polypropylene, perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), and polytetrafluoroethylene (PTFE) polymers are typical pipe lining materials.
Joining sections of lined pipe can be done by a variety of methods, including using flanged pipe sections and welding the pipe sections together. It is preferred to weld the pipe liners together in order to form a secure pipe joint which is less likely to leak than a flanged pipe joint.
However, when fluorinated polymeric resins are utilized as lining materials, such as PFA, FEP, and PTFE, conventional welding techniques have proven ineffective, due to, for example, the difficulty in welding melt-processable PFA and FEP to themselves using contact welding techniques, and fusing sintered or ram extruded PTFE to itself due to the lack of adhesion even when heat and pressure are employed. An attempt to overcome the adhesion problems related to joining PTFE articles is described in U.S. Pat. 3,946,136, assigned to Hoechst Aktiengesellschaft. Yet, the '136 patent describes heating the resin from the outside in rather than heating the PTFE surfaces themselves and describes using an oven. Thus, the invention described therein cannot be used in the field. Another attempt to overcome adhesion problems related to joining PTFE articles is described in U.S. Pat. No. 4,073,856, assigned to Resistoflex; however, the '856 patent describes the use of a complicated mold which does not have the capability of being used in the field.
Hot plate welding techniques have been successfully used in the field. However, hot plate welding techniques have been unsuccessful for use in welding fluorinated melt-processable materials, because the fluoropolymeric materials have adhered to the hot plate welding equipment.
Non-contact welding techniques have also been used. Such techniques utilize infrared welding energy sources which heat the exterior and also penetrate the surface and heat the interior of the polymeric article. This non-contact approach overcomes the undesirable adhesion problem of the contact heater; however, this non-contact method allows the polymeric liners to relieve residual stress from the molding process and/or extrusion process, and any additional stresses incurred during the manufacturing process. This stress relieving, associated with the penetrating heat of the infrared source, causes the ends of the liners to geometrically distort, resulting in a less than desirable weld where the liners are joined together. This non-contact method is also difficult to use in the field.
It would be an advance in the art of plastic lined pipe products to provide a simple apparatus and method for welding fluorinated polymeric pipe liners together which is capable of being used in the field.